Added missing failure code to charge entity

While using stripe.net I noticed it was missing the FailureCode property on the Charge object. I needed the failure code to know when a charge failed for an invoice so I can let the user know the issue with the payment for their invoice. I'm not sure why it wasn't there or if Stripe recently added it. I also added a test to make sure it was working properly.

Here is the documentation for the charge object: https://stripe.com/docs/api#charge_object